The present invention relates to a catalyst in extruded form which is obtained by mixing a high-viscosity sol obtained from the hydrolysis and polycondensation of silicates and aluminates, with an inert binding agent.
The invention relates as well to the use of such a catalyst in olefin oligomerization processes.
Some silica/alumina gels, of amorphous character, displaying catalytic activity, are known in the art. So, e.g., in EP 160,145 a process for alkylating aromatic hydrocarbons is disclosed, which uses a catalyst consisting of a silica/alumina gel, of amorphous character, having pores with a diameter typically comprised within the range of from 50 to 500 xc3x85ngstroms, and with a ratio of silica to alumina typically, comprised within the range of from 1:1 to 10:1.
M. R. S. Manton and J. Davidtz in Journal of Catalysis, 60, 156-166 (1979) describe a process for the synthesis of amorphous silica/alumina catalysts having a controlled pore volume, i.e., typically comprised within the range of from 3.7 to 15 nm.
European patent application EP 340,868 discloses a silica/alumina gel, amorphous when examined by X-ray analysis, having a molar ratio of SiO2/Al2O3 of from 30:1 to 500:1, with a specific surface area comprised within the range of from 500 to 1000 m2/g, a total pore volume of from 0.3 to 0.6 ml/g, and substantially free from pores with larger diameter than 30 xc3x85ngstroms.
This silica-alumina gel is prepared as follows:
(a) an aqueous solution is prepared of a tetraalkyl ammonium hydroxide (TAAxe2x80x94OH), a soluble aluminum compound capable of yielding Al2O3 by hydrolysis and a silicon compound capable of yielding SiO2 by hydrolysis, in the following mutual molar ratios:                                           SiO            2                    :                                    Al              2                        ⁢                          O              3                                      =                                            from                        ⁢                          xe2x80x83                        ⁢            30                    :                                    1              ⁢                              xe2x80x83                            ⁢                              to                            ⁢                              xe2x80x83                            ⁢              500                        :            1                                                            TAAxe2x80x94OH          :                      SiO            2                          =                                            from                        ⁢                          xe2x80x83                        ⁢            0.05                    :                                    1              ⁢                              xe2x80x83                            ⁢                              to                            ⁢                              xe2x80x83                            ⁢              0.2                        :            1                                                                                      H              2                        ⁢                          O              :                              SiO                2                                              =                                                    from                            ⁢                              xe2x80x83                            ⁢              5                        :                                          1                ⁢                                  xe2x80x83                                ⁢                                  to                                ⁢                                  xe2x80x83                                ⁢                40                            :              1                                      ;            
(b) the resulting solution is heated in order to cause it to undergo hydrolysis and gelation;
(c) the resulting gel is dried;
(d) the dried gel is calcined, firstly under an inert atmosphere and then under an oxidizing atmosphere.
The resulting silica/alumina gel is catalytically active in hydrocarbon conversion processes.
Of course, the problem existed of rendering the silica/alumina gel disclosed in the above said patent application best suitable for industrial use, by endowing it with adequate properties of mechanical strength, without endangering the high catalytic performance thereof.
Those skilled in the art are aware of the possible procedures for preparing extruded bodies having high enough mechanical strength values, without altering their catalytic performance. Thus, for example, the catalyst can be ground, so as to obtain powders consisting of particles with a suitable size, which are subsequently blended with a thickener.
A further preparation method consists in blending silica/alumina gel powders with a second powder of a metal oxide in the presence of a thickening agent.
All these techniques yield extrudates endowed with good mechanical strength and unchanged catalytic performance.
In EP 550,922 an extruded catalyst is disclosed which is prepared according to known techniques, and consists of:
a catalytically active portion constituted by the silica/alumina gel disclosed in EP 340,868;
an inert binding agent constituted by aluminas belonging to the class of boehmite or pseudo-boehmite.
This catalyst displays a good mechanical strength, and results to be more active than pristine silica/alumina gel.
Said catalyst is prepared by blending the catalytically active portion constituted by silica/alumina gel, suitably ground in order to obtain a powder with a smaller average diameter than 50 microns, with the inert binding agent, in the presence of a thickener containing a mineral or organic acid, until a homogeneous paste is obtained which is then extruded yielding small cylindrical bodies of catalyst which are submitted to ageing, drying at 100-120xc2x0 C. and calcination in air at a temperature comprised within the range of from 500 to 600xc2x0 C.
The present Applicants found now unexpectedly, that by adding boehmite or pseudo-boehmite during that silica/alumina gel preparation step which precedes the drying and calcination steps, a catalyst is obtained which is more active than pristine gel, and the extruded catalyst disclosed in EP 550,922, based on a silica/alumina gel and a binding agent selected from boehmite or pseudo-bohemite.
Furthermore, the catalyst according to the present invention requires a lower number of synthesis steps than as required in order to prepare the extruded catalyst of EP 550,922: in particular, the step of grinding the dried/calcined gel is no longer necessary, and the drying and calcination steps are carried out once only, on the extruded catalyst.
Therefore, the object of the present invention is a catalyst in extruded form, consisting of an inert binder agent and a catalytically active portion of silica/alumina gel, which is obtained by:
(a) preparing an aqueous solution of a tetraalkyl ammonium hydroxide (TAAxe2x80x94OH), a soluble aluminum compound capable of yielding Al2O3 by hydrolysis and a silicon compound capable of yielding SiO2 by hydrolysis, in the following mutual molar ratios:                                           SiO            2                    :                                    Al              2                        ⁢                          O              3                                      =                                            from                        ⁢                          xe2x80x83                        ⁢            30                    :                                    1              ⁢                              xe2x80x83                            ⁢                              to                            ⁢                              xe2x80x83                            ⁢              500                        :            1                                                            TAAxe2x80x94OH          :                      SiO            2                          =                                            from                        ⁢                          xe2x80x83                        ⁢            0.05                    :                                    1              ⁢                              xe2x80x83                            ⁢                              to                            ⁢                              xe2x80x83                            ⁢              0.2                        :            1                                                                                      H              2                        ⁢                          O              :                              SiO                2                                              =                                                    from                            ⁢                              xe2x80x83                            ⁢              5                        :                                          1                ⁢                                  xe2x80x83                                ⁢                                  to                                ⁢                                  xe2x80x83                                ⁢                40                            :              1                                      ;            
(b) heating the resulting solution in order to cause the reaction mixture to undergo hydrolysis and gelation, and obtain an (A) mixture having a viscosity comprised within the range of from 0.01 to 100 Paxc2x7sec;
(c) adding to said (A) mixture, firstly a binding agent selected from the class of boehmites or of pseudo-boehmites, in a ratio, by weight, to said (A) mixture comprised within the range of from 0.05 to 0.5; and then a mineral or organic acid, in an amount comprised within the range of from 0.5 to 8 g per 100 g of binding agent;
(d) mixing and heating up to a temperature comprised within the range of from 40 to 90xc2x0 C., the resulting mixture from above (c) step, until a homogeneous paste is obtained, which is submitted to extrusion;
(e) the resulting extrudate is dried;
(f) the dried extrudate is calcined under an oxidizing atmosphere.
The composition of the mixture from the (a) step and the nature of the reactants used are in accordance with the teachings of EP 340,868.
The (b) step is carried out at a temperature comprised within the range of from 60 to 100xc2x0 C., during a time comprised within the range of from 15 minutes to 2 hours. Reaching a viscosity of from 0.01 to 100 Paxc2x7sec is critical in this step. More viscous, or less viscous products are unsuitable for those treatments according to the subsequent processing steps.
In the (c) step, the binding agent is preferably used in powder form, with an average particle diameter of less than 50 xcexcm.
According to a preferred aspect of the present invention, in the (c) step also a plasticizer is added. The plasticizer can be, e.g., methyl cellulose, stearin, glycerol. The plasticizer is added at an intermediate time between the addition of the binding agent and the addition of mineral or organic acid.
In the (d) step, the mechanical mixing and heating cause the solvent to evaporate until a homogeneous paste is obtained which has such a consistency as normally regarded as suitable for the extrusion. Cylindical catalyst bodies are obtained with sizes which may be varied as a function of application requirements, and are then submitted to ageing at a temperature comprised within the range of from 20 to 40xc2x0 C.
In the (e) step, the catalyst is submitted to an oven-drying at 100-120xc2x0 C., and then, in the (f) step, to a calcination in air, at a temperature comprised within the range of from 500 to 600xc2x0 C.
The resulting catalyst has a higher catalytic activity than pristine silica/alumina gel, as well as than silica/alumina gel bound, according to the well-known methods, with bohemite or pseudo-bohemite.
This catalyst is furthermore well useable at an industrial level, because it displays an axial breaking strength comprised within the range of from 25 to 280 kg/cm2, a bimodal porosity distribution and a specific surface area comprised within the range of from 400 to 600 m2/g.
The catalyst of the present invention can be suitably used in the usual acid-catalysed petrochemical reactions, as alkylation, isomerization and oligomerization.
In particular, it is very effective in the reaction of oligomerization of light olefins, in particular propylene, to yield hydrocarbon cuts displaying extremely good properties as gasoline and jet fuel.
Said oligomerization is suitably carried out at a temperature comprised within the range of from 100 to 250xc2x0 C. and under a pressure of from 10 to 70 bars.